1. Field of the Invention
This invention relates to a technique for driving a liquid crystal display, and more particularly to an apparatus and method for correcting a gamma voltage and a video data in a liquid crystal display that is capable of improving a display quality of the liquid crystal display.
2. Description of the Related Art
Generally, an active matrix liquid crystal display (LCD) uses thin film transistors (TFTs) as switching devices to display a natural moving picture. Since such a LCD can be made into a smaller device in size than the existent Brown tube, it has been widely used for a monitor for a personal computer or a notebook computer as well as office automation equipment such as copy machines, etc. and portable equipment such as cellular phones and pagers, etc.
As shown in FIG. 1, a driving apparatus for the LCD includes a digital video card 1 for converting an analog signal into a digital video data, a column driver 3 for applying the video data to data lines DL of a liquid crystal panel 6, a row driver 5 for sequentially driving gate lines GL of the liquid crystal panel 6, a controller 2 for controlling the column driver 3 and the row driver 5, and a gamma voltage generator 4 for applying a gamma voltage to the column driver 3.
In the liquid crystal panel 6, liquid crystal is injected between two glass substrates, and the gate lines GL and the data lines DL are formed on the lower glass substrate in such a manner as to be perpendicular to each other. At each intersection between the gate lines GL and the data lines DL, a thin film transistor (TFT) for selectively applying an image input from the data lines DL to a liquid crystal cell Clc is provided. To this end, the TFT has a gate terminal connected to the gate line GL and a source terminal connected to the data line DL. The drain terminal of the TFT is connected to a pixel electrode of the liquid crystal cell Clc.
The digital video card 1 converts an analog input image signal into a digital image signal suitable for the liquid crystal panel 6 and detects a synchronous signal included in the image signal. The controller 2 applies red (R), green (G) and blue (B) digital video data from the digital video card 1 to the column driver 3. Also, the controller 2 generates a dot clock Dclk and a gate start pulse GSP using horizontal/vertical synchronizing signals H and V input from the digital video card 1 to provide a timing control of the column driver 3 and the row driver 5. The dot clock Dclk is applied to the column driver 3 while the gate start pulse GSP is applied to the row driver 5.
The row driver 5 includes a shift register for responding to the gate start pulse GSP input from the controller 2 to sequentially generate a scanning pulse, and a level shifter for shifting a voltage of the scanning pulse to a voltage level suitable for driving the liquid crystal cell. Video data at the data line DL is applied to a pixel electrode of the liquid crystal cell Clc by the TFT in response to the scanning pulse input from the row driver 5.
The dot clock Dclk, along with the R, G and B digital video data from the controller 2, is input to the column driver 3. The column driver 3 latches the R, G and B digital video data in synchronization with the dot clock Dclk and corrects the latched data in accordance with a gamma voltage Vγ. Then, the column driver 3 converts data corrected by the gamma voltage Vγ into analog data and supplies it to the data line DL for each line.
As shown in FIG. 2, the column driver 3 includes a first latch 21 to which R, G and B data are input, a second latch 22, a digital to analog converter (DAC) 23 and an output buffer 24 connected, in series, between the first latch 21 and the data lines DLl to DLn, and an address shift register 25 for assigning an address of the second latch 25.
The first latch 21 temporarily stores the R, G and B data from the controller 2 and applies the stored data to the second latch 22 every horizontal period. The second latch 22 stores data from the first latch 21 in a location indicated by address information from the address shift register 25 and supplies the stored data for one line to the DAC 23.
The DAC 23 selects a gamma voltage Vγ corresponding to data from the second latch 22 and applies it to the data lines DLl to DLn. A detailed description as to this DAC 23 will be made in conjunction with FIG. 6 later. The output buffer 24 consists of a voltage follower connected in series to the data line DL so as to buffer data from the DAC 23 and apply the buffered data to the data lines DLl to DLn. The output buffer 24 and the second latch 22 receive a polarity inverting signal from the controller 2 for the purpose of inverting the polarity of the video data depending on an inversion driving system, such as a dot inversion system, a line (or column) inversion system, and a frame inversion system.
The address shift register 25 generates address information for the data stored in the second latch 22 to control the second latch 22. The gamma voltage generator 4 generates a gamma voltage Vγ corresponding to a gray level value of data, in consideration of an electro-optical characteristic of the liquid crystal panel 6, and applies it to the DAC 23. The gamma voltage Vγ from the gamma voltage generator 4 is set to have a different voltage magnitude in correspondence with a gray level value selected in an expressible range as shown in FIG. 3. In FIG. 3, in the normally white mode, data having the lowest brightness is GMA1, corresponding to a voltage Vdd, and data having a relatively higher brightness corresponds to GMA2, GMA3, . . . , GMAN.
Each liquid crystal cell Clc expresses a gray level value having a specific brightness by a relative potential difference between the gamma voltage Vγ and a common voltage Vcom. More specifically, as shown in FIG. 4, an LCD with the normally white mode expresses an image at a brightness close to white when a potential difference between the gamma voltage Vγ and the common voltage Vcom is low, whereas it expresses an image at a brightness gradually closer to black as a potential difference between the gamma voltage Vγ and the common voltage Vcom becomes high. When a gamma voltage Vγ corresponding to an input image signal data expressed by a hexadecimal digit is selected, an analog voltage as shown in FIG. 5 is applied to the liquid crystal cell Clc of the liquid crystal panel 6. The gamma voltage generator 4 is classified into a positive part and a negative part to correspond to the inversion driving system. A configuration of the positive part is as shown in FIG. 6. The negative part has a configuration substantially identical to the positive part except for the polarity of a supplied voltage.
Referring to FIG. 6, the positive part type gamma voltage generator 4 includes: a reference voltage generator 41 for generating reference voltages VH1 to VH6 each having a different voltage level in accordance with a voltage-divided resistance ratio; a buffer unit 42 connected to an output terminal of the reference voltage generator 41; and a gamma voltage output 43 connected between the buffer unit 42 and the DAC 23 to divide the reference voltage VH1 to VH6 and output gamma voltages Vγ having different voltage levels.
The reference voltage generator 41 includes a serial connection of first to sixth resistors R1 to R6 to generate six reference voltages VH1 to VH6 in accordance with a voltage-divided resistance ratio, and to apply them to the buffer unit 42. The buffer unit 42 consists of a voltage follower connected, in series, between an output terminal of the reference voltage generator 41 and the gamma voltage output 43. The buffer unit 42 stabilizes the reference voltages VH1 to VH6 and applies them to the gamma voltage output 43. The gamma voltage output 43 consists of a serial connection of 64 resistors R11 to R164. The gamma voltage output 43 sub-divides the six reference voltages VH1 to VH6 into 64 gamma voltages and applies them to the DAC 23.
The DAC 23 includes a data input 44 for receiving 6-bit data D0 to D5 from the second latch 22, and a decoder 45 connected between the data input 44 and the gamma voltage output 43. The data input 44 includes an inverter for inverting a logical value of each data bit to generate an inverted signal and a non-inverted signal of the data and to apply them to the decoder 45. The decoder 45 consists of a plurality of logical elements in an array to select any one of the 64 gamma voltages Vγ in accordance with the inverted and non-inverted data from the data input 44 and to apply the selected gamma voltage Vγ to the output buffer 24.
Nowadays, the LCD requires interchangeability with various peripheral equipment capable of displaying image signals input from a personal computer, a television, a player for an optical recording medium such as a compact disk (CD) or a digital versatile disk (DVD), or a camcoder, etc. However, the conventional driving apparatus for the LCD cannot correct a gamma voltage enough to be suitable for each image signal from the various peripheral equipment because the gamma voltage has been fixed by a predetermined voltage-divided resistance ratio. As a result in the case of displaying an image signal inputted from the peripheral equipment, the conventional LCD presents color distortion, etc., of a displayed image, depending on the type of the peripheral equipment to thereby cause a deterioration in quality of the displayed image.
Also, the conventional LCD has a problem in that, since it has a poor correlative color temperature, it cannot obtain constant chrominance co-ordinates in accordance with a value of the input data. In other words, as can be seen from the color co-ordinates of FIG. 7 that is indicated by the XYZ system defined by the Committee International Ellumination (CIE), the LCD has a serious variation in a correlative color temperature because it has a wide and irregular correlative color temperature distribution. If a variation in the correlative color temperature is serious, it becomes difficult to provide a color expression corresponding to a desired gray level value for a black and white image as well as for a color image and hence a displayed image becomes unnatural.
In FIG. 7, the horizontal axis and the vertical axis represent independent parameters x and y, respectively, when a color is displayed by the CIE co-ordinate system. The solid line indicates a color temperature of an ideal blackbody emitting a light identical to a light from a light source. In FIG. 7, “•” represents a correlative color temperature according to a gray level value of an input image. D65 represents a standard light source corresponding to sunshine in broad daylight in which a correlative color temperature is 6504 K, whereas C represents a standard light source corresponding to average sunshine on a cloudy day in which a correlative color temperature is 6774 K. In reality, since only video data corresponding to the highest brightness in the LCD has an appropriate color temperature value, a real image is observed at a white level. However, a real image is observed at a blue color because a correlative color temperature is considerably high when a digit value of a video data is small, that is, when it is dark, whereas it is observed at a slight blue color in the case of a video data digit value having a middle brightness. As a result, since the screen is observed with a bluish color as a whole, it becomes difficult to provide a natural color display. This is caused by physical and optical characteristics of a liquid crystal. There is a limit in solving such a problem by a correction of the gamma voltage.